1. Field of the Technology
The present disclosure is applicable to methods and systems of using reflective optical sensors to monitor very low densities of toner on various substrates within a print engine. More particularly, the present disclosure relates to process controls and system based on the amount of toner residual mass remaining on a substrate after various image transfer operations such as transfer of the image to another medium.
2. Description of the Prior Art
Xerographic process control systems often use reflective optical sensors to monitor the amount of toner present on various substrates or surfaces at different points during the printing process. For example, ETAC (Enhanced Toner Area Concentration) sensors and FWA (full-width array) sensors can be used to monitor the amount of toner present on an intermediate transfer belt (ITB) or on a photoreceptor (P/R) belt. While these sensors work well for detecting large amounts of toner—such as the developed mass before the 2nd transfer system—careful calibration is required to make these sensors yield meaningful data for small amounts of toner, such as the residual mass left behind after the 2nd transfer point. In particular, in order to accurately measure residual mass after 2nd transfer, a careful measurement of the reflectance profile of the bare belt is required. The reason for this is that the measured reflectance signal is dominated by the reflectance profile of the bare substrate. The signal of interest is, in fact, the deviation from this bare-belt reflectance signal that is caused by the presence of the residual mass. Thus, without the bare belt signature, the data for the residual mass can not be adjusted correctly to extract the true signal, and an erroneous mass reading would result. Although the main context for this invention is the measurement of low toner masses (e.g., residual mass), the methodology is still applicable to higher toner masses (e.g., developed mass). This is because the typical algorithm for calculating developed mass actually compares the sensor's data signal to the bare belt signature (the essence of the present invention). The improvement in accuracy in this case, may be smaller, however, the same techniques of the present invention apply.
Currently, the most common method for measuring the bare belt signature is to print a blank job containing 10 or 20 pages (which would correspond to multiple revolutions of the belt loop), while monitoring the output of the optical sensor. The data is then averaged point by point across the multiple belt revolutions to reduce the noise in the signal. This produces a mean once-around bare belt signature. Alternatively, if the capability exists, the belt substrate could be rotated alone, without the need to feed extra paper through the printer. However, this does not change the amount of time that would be needed to perform this measurement. Furthermore, when executing a large set of print jobs, the belt signature will need to be re-measured periodically to take into account drifts in temperature, drifts in sensor parameters (e.g., changes in the illumination source), filming of the belt, and other systematic shifts in the process under test. This type of bare belt signature drift on the resultant measured residual mass signal has been demonstrated. An “out of date” bare belt signature can have a significant effect on the measurement of residual toner mass.
This requirement for frequent recalibration of the bare belt signature results in an even larger number of wasted pages and/or belt cycles, and (more importantly) wasted time, making the calibration process very time-consuming for actual product-intent control systems. A rough estimate for some machines or systems is that a re-calibration of the bare belt signature would be needed every 100-200 pages. Assuming that 10 pages are used for the calibration process, this would represent a 5-10% drop in productivity.
Thus, in accordance with the present invention, a faster (no extra belt cycles) and potentially more accurate measurement than previous methods is achieved. This is because the reflectance profile of the xerographic image surface is measured during the sensing time period itself, as opposed to being measured before or after the sensing period. This significantly reduces the time required for the reflectance profile measurement, as well as eliminating the time lag problem that exists in other methods for performing this measurement. In addition, unlike other methods, this technique does not require the use of a custom print target, such as the one described, for example, by Hamby et al. in U.S. Pat. No. 7,120,369.
These advantages allow the bare belt signature to be re-measured on a regular basis, which greatly improves the measurement accuracy of the sensor, particularly for low densities of toner. This is especially useful for performing in-situ measurements of residual mass on customer print jobs, which may in turn lead to better process controls and better image quality in the final prints.